Reactivity, Structures, and NMR Spectroscopy of Half-Sandwich

Organometallics 2009, 28, 1435–1446

1435

Reactivity, Structures, and NMR Spectroscopy of Half-Sandwich Pentamethylcyclopentadienyl Rhodium Amido Complexes Relevant to Transfer Hydrogenation A. John Blacker,† Simon B. Duckett,‡ Jake Grace,‡ Robin N. Perutz,*,‡ and Adrian C. Whitwood‡ Piramal Healthcare, Leeds Road, Huddersfield, U.K., HD1 9GA, and Department of Chemistry, UniVersity of York, York, U.K., YO10 5DD ReceiVed October 15, 2008

The reactivity, structures, and NMR spectroscopy of a series of compounds relevant to asymmetric transfer hydrogenation, of general formula Cp*RhCl(S,S-4-RC6H4SO2NCHPhCHPhNH2) (Cp* ) η5C5Me5, S,S-2a R ) Me, S,S-2b R ) tBu, S,S-2c R ) F), have been studied. 1H/15N HMQC NMR spectra of 2a-2c were recorded with 15N in natural abundance making use of the coupling to the C5Me5 protons; the coupling constants JRhN were ca. 15 and 20 Hz for the amino and amido nitrogens, respectively. 1 H/103Rh NMR spectra of 2a and 2c were recorded similarly. The chloride ligand in S,S-2a has been shown to be very labile; reaction with CO afforded the cationic complex [Cp*Rh(CO){C(O)N(Ts)CHPhCHPhNH2}][Cl] (3a) as a mixture of diastereomers, S,S,RRh and S,S,SRh, with opposite chirality at Rh; reaction with tBuNC gave [Cp*Rh(CNtBu)(S,S-TsCHPhCHPhNH2)][PF6] (4a), and reactions with LiBr and KI gave Cp*RhBr(S,S-TsNCHPhCHPhNH2) (6a) and Cp*RhI(S,S-TsNCHPhCHPhNH2) (7a), respectively. Complexes S,S-2c, S,S-6a, and S,S-7a have been characterized by X-ray crystallography; the amino N-Rh bond is significantly shorter than the amido N-Rh bond in all cases (difference ∆r 0.058(2), 0.085(6), and 0.046(4) Å, respectively), and the complexes all possess intramolecular NH · · · X (X ) Cl, Br, I) hydrogen bonds. The reaction of 2a with formic acid results in complete displacement of the chelating ligand and the formation of dinuclear [{Cp*Rh}2(µ-H)(µ-Cl)(µ-HCO2)][BPh4] (5a), which was also characterized crystallographically. Reaction of 2c with AgOTf resulted in the formation of [Cp*Rh(OH2)(4-FC6H4SO2NCHPhCHPhNH2)][OTf]([8c · H2O][OTf])or[Cp*Rh(4-FC6H4SO2NCHPhCHPhNH2)]2[OTf]2 ([8c]2[OTf]2) depending upon the conditions employed. [8c · H2O][OTf] was characterized by X-ray crystallography, and the structure showed that both CHPh centers in the ligand had been racemized, converting the S,S isomer of the starting material into a mixture of both R,R and S,S isomers. Introduction Asymmetric transfer hydrogenation catalyzed by halfsandwich complexes with monotosylated diamine ligands was described first by Noyori et al.1,2 The mechanism for the transfer hydrogenation of CO double bonds with iPrOH/KOH as the hydrogen donor has been described as outer-sphere metal-ligand bifunctional catalysis;3 a schematic for the [(p-cymene)RuCl2]2/ TsDPEN (TsDPEN ) N-tosyl 1,2-diphenylethylenediamine) system is shown in Figure 1. First, [(p-cymene)RuCl2]2 and TsDPEN react to form the catalyst precursor (step 1). The precursor then reacts with base (KOH) to give a 16-electron species (step 2), which is hydrogenated by iPrOH (the hydrogen donor), leading to an 18-electron hydride complex (step 3). The latter transfers its hydride to the carbonyl carbon of the substrate and a proton from its amino nitrogen to the oxygen of the substrate, to give the product alcohol and re-form the 16-electron species (step 4). Step 4 is proposed to transfer hydrogen via a

Figure 1. Proposed catalytic cycle for outer-sphere metal-ligand bifunctional catalysis.3

* Corresponding author. E-mail: [email protected]. † Piramal Healthcare. ‡ University of York. (1) Hashiguchi, S.; Fujii, A.; Takehara, J.; Ikariya, T.; Noyori, R. J. Am. Chem. Soc. 1995, 117, 7562. (2) Noyori, R.; Hashiguchi, S. Acc. Chem. Res. 1997, 30, 97. (3) Haack, K. J.; Hashiguchi, S.; Fujii, A.; Ikariya, T.; Noyori, R. Angew. Chem., Int. Ed. 1997, 36, 285.

six-membered, pericyclic transition state, without coordination of the substrate to Ru. Catalytic transfer hydrogenation of CdN bonds using HCO2H and its salts as the hydrogen donor4-12 was first reported by Noyori and co-workers in 1996.4,13 Due to its operational simplicity and wide substrate scope, it has become the method of choice for the synthesis of many chiral amines.

10.1021/om8009969 CCC: $40.75  2009 American Chemical Society Publication on Web 02/09/2009

1436 Organometallics, Vol. 28, No. 5, 2009

Noyori and co-workers have isolated and characterized all three ruthenium complexes in Figure 1 by X-ray crystallography and have shown that they all reduce acetophenone with comparable activity and selectivity.3 Following this, attempts were made to study the behavior of the analogous Cp*M(III) (M ) Rh or Ir, Cp* ) η5-C5Me5) systems;14-17 however, these systems are generally more active and the analogous intermediates have proved harder to isolate (the rhodium and iridium systems are also described in patents belonging to Piramal Healthcare).18-20 Tani et al. showed that [Cp*RhCl2]2 and TsDPEN react with NaOH to give the 16-electron species Cp*Rh(TsNCHPhCHPhNH),14 but the rhodium hydride complex proved elusive. For iridium, a bis-amido 16-electron species was obtained as described above for rhodium; the same product was obtained by reaction of the chloride precursor with aqueous KOH in CH2Cl2. The hydride Cp*Ir(H)(TsNCHPhCHPhNH2) was then prepared by reaction of this 16-electron complex with iPrOH. The reverse reaction of the hydride complex with acetone gave back the bis-amido complex.14 Ikariya and co-workers reported the synthesis of the related complexes Cp*RhCl(TsNC6H10NH2) (where C6H10 is 1,2disubstituted cyclohexane), Cp*RhCl(TsNCHPhCHPhNH2), Cp*IrCl(TsNC6H10NH2), and Cp*IrCl(TsNCHPhCHPhNH2), which also act as catalyst precursors for the asymmetric transfer hydrogenation of acetophenones.15,16 They showed that the reaction of Cp*IrCl(TsNC6H10NH2) with 1 equiv of NaOH in iPrOH gave quantitative conversion to the 18-electron hydride Cp*Ir(H)(TsNC6H10NH2), which then reacted with acetone to give the bis-amido complex Cp*Ir(TsNC6H10NH). Recently, Heiden and Rauchfuss exploited the interconversion of Cp* Ir(TsNCHPhCHPhNH), [Cp*Ir(TsNCHPhCHPhNH2)]+, and Cp*Ir(H)(TsNCHPhCHPhNH2), to demonstrate that the hydride complex acts as a catalyst for the conversion of dihydrogen and dioxygen to water.21 They also showed that the 16-electron cation [Cp*Ir(TsNCHPhCHPhNH2)]+ reacts with Lewis bases to form simple adducts [Cp*Ir(TsNCHPhCHPhNH2)L]+ (L ) MeCN, NH3, PAr3, and CO).22

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In spite of the successful synthesis of these 16-electron and hydride species, there has still been considerable debate about their role in the transfer hydrogenation of imines. Evidence from isotope labeling supports a monohydride mechanism in which the hydride and the proton undergoing transfer remain distinct for Rh, Ir, and Ru catalysts.23-25 On the other hand, it has been much harder to define when the outer-sphere (Noyori mechanism) occurs and when a more conventional inner-sphere mechanism occurs in which the substrate binds directly to the metal.11,26-33 A better understanding of the chemistry of complexes such as Cp*RhCl(TsNCHPhCHPhNH2) (2a) will surely aid our understanding of the catalytic systems, since 2a is generated in situ in catalytic asymmetric transfer hydrogenation reactions (the process is named CATHy, catalytic asymmetric transfer hydrogenation).18-20 It also catalyzes the asymmetric Michael addition reaction34,35 and reacts with sodium phenoxide in dichloromethane, albeit very slowly and in low yield, to give a cyclometalated product.36 Our aim was to study the reactivity of the catalyst 2a with a focus on the identification of complexes relevant to transfer hydrogenation. Because of the poor solubility of the complexes derived from N-tosyl-1,2-diphenylethylenediamine (TsDPEN) (1a), we synthesized two additional ligand precursors, 4-tBuC6H4SO2DPEN) (1b) and 4-FC6H4SO2DPEN (1c). The resulting metal complexes were more soluble than those of 1a, and the F atom in 1c provided an additional NMR handle. We describe the reactions with CO, tBuNC, and AgOTf leading to cationic complexes. We show that the reaction of 2a with HCO2H displaces the chelating chiral ligand completely. We also include the metathesis reactions that allow replacement of chloride with bromide and iodide. Throughout this work, we place emphasis on structural features including intramolecular hydrogen bonding (as noted previously)3,5,15 that we investigate through X-ray crystallography and NMR spectroscopy. We have also recorded 15 N and 103Rh NMR spectra as additional probes.

Results and Discussion (4) Hashiguchi, S.; Uematsu, N.; Noyori, R. J. Synth. Org. Chem. Jpn. 1997, 55, 99. (5) Mao, J. M.; Baker, D. C. Org. Lett. 1999, 1, 841. (6) Chrzanowska, M.; Rozwadowska, M. D. Chem. ReV. 2004, 104, 3341. (7) Vedejs, E.; Trapencieris, P.; Suna, E. J. Org. Chem. 1999, 64, 6724. (8) Meuzelaar, G. J.; van Vliet, M. C. A.; Maat, L.; Sheldon, R. A. Eur. J. Org. Chem. 1999, 2315. (9) Kaldor, I.; Feldman, P. L.; Mook, R. A.; Ray, J. A.; Samano, V.; Sefler, A. M.; Thompson, J. B.; Travis, B. R.; Boros, E. E. J. Org. Chem. 2001, 66, 3495. (10) Roszkowski, P.; Wojtasiewicz, K.; Leniewski, A.; Maurin, J. K.; Lis, T.; Czarnocki, Z. J. Mol. Catal. A: Chem. 2005, 232, 143. (11) Samec, J. S. M.; Ba¨ckvall, J.-E.; Andersson, P. G.; Brandt, P. Chem. Soc. ReV. 2006, 35, 237. (12) Clapham, S. E.; Hadzovic, A.; Morris, R. H. Coord. Chem. ReV. 2004, 248, 2201. (13) Uematsu, N.; Fujii, A.; Hashiguchi, S.; Ikariya, T.; Noyori, R. J. Am. Chem. Soc. 1996, 118, 4916. (14) Mashima, K.; Abe, T.; Tani, K. Chem. Lett. 1998, 1201. (15) Murata, K.; Ikariya, T.; Noyori, R. J. Org. Chem. 1999, 64, 2186. (16) Ikariya, T.; Blacker, A. J. Acc. Chem. Res. 2007, 40, 1300. (17) Mashima, K.; Abe, T.; Tani, K. Chem. Lett. 1998, 1199. (18) Blacker, A. J. Handbook of Homogeneous Hydrogenation; WileyVCH:: New York, 2007. (19) Blacker, A. J.; Mellor, B. EP971865B1, 1997. (20) Blacker, A. J.; Martin, J. Scale up Studies in Asymmetric Transfer Hydrogenation. In Asymmetric Catalysis on Industrial Scale; Blaser, H.U.; Schmidt, E., Eds.; Wiley-VCH: Weinheim, 2004; p 201. (21) Heiden, Z. M.; Rauchfuss, T. B. J. Am. Chem. Soc. 2007, 129, 14303. (22) Heiden, Z. M.; Gorecki, B. J.; Rauchfuss, T. B. Organometallics 2008, 27, 1542.

Synthesis and Structure of Cp*RhCl(4-RC6H4SO2NCHPhCHPhNH2), 2a-2c. The N-aromatic sulfonyl-1,2-diphenylethylenediamine ligands were synthesized by the method of White et al. (Scheme 1).37 Both the R,R and S,S isomers of 1b and 1c were prepared, but we show the S,S isomers in the schemes. The ligands 1a-1c were complexed to rhodium by reaction with [Cp*RhCl2]2 to yield Cp*RhCl(4-RC6H4SO2NCHPhCHPhNH2) [2a (R ) Me), 2b (R ) tBu), and 2c (R ) F)]. Complex (23) Samec, J. S. M.; Ell, A. H.; Åberg, J. B.; Privalov, T.; Eriksson, L.; Ba¨ckvall, J.-E. J. Am. Chem. Soc. 2006, 128, 14293. (24) Casey, C. P.; Singer, S. W.; Powell, D. R.; Hayashi, R. K.; Kavana, M. J. Am. Chem. Soc. 2001, 123, 1090. (25) Casey, C. P.; Johnson, J. B. J. Org. Chem. 2003, 68, 1998. (26) Samec, J. S. M.; Ell, A. H.; Ba¨ckvall, J.-E. Chem. Commun. 2004, 2478. (27) Ell, A. H.; Johnson, J. B.; Ba¨ckvall, J.-E. Chem. Commun. 2003, 1652. (28) Casey, C. P.; Johnson, J. B. J. Am. Chem. Soc. 2005, 127, 1883. (29) Casey, C. P.; Beetner, S. E.; Johnson, J. B. J. Am. Chem. Soc. 2008, 130, 2285. (30) Privalov, T.; Samec, J. S. M.; Ba¨ckvall, J.-E. Organometallics 2007, 26, 2840. (31) Johnson, J. B.; Ba¨ckvall, J.-E. J. Org. Chem. 2003, 68, 7681. (32) Casey, C. P.; Bikzhanova, G. A.; Cui, Q.; Guzei, I. A. J. Am. Chem. Soc. 2005, 127, 14062. (33) Casey, C. P.; Johnson, J. B.; Singer, S. W.; Cui, Q. J. Am. Chem. Soc. 2005, 127, 3100. (34) Watanabe, M.; Murata, K.; Ikariya, T. J. Am. Chem. Soc. 2003, 125, 7508.

Half-Sandwich Rhodium Amido Complexes

Organometallics, Vol. 28, No. 5, 2009 1437

Scheme 1. Synthesis of 1a-1c

2a has been described previously.5,15,17,19 Complexes 2a-2c were prepared by treatment of [Cp*RhCl2]2 with 1a-1c and NEt3 in CH2Cl2 and were crystallized as red-orange needles. The most interesting features of the 1H NMR spectra of 2a-2c are the chemical shift changes for the NH2 protons that were observed when the solvent was changed. For 2a, the NH protons are observed at δ 3.46 and 3.88 in CD2Cl2, but are shifted to δ 4.42 and 3.60 in CD3CN (Figure 2). This is consistent with intramolecular hydrogen bonding in CD2Cl2, a weak H-bond acceptor, and a more important role for intermolecular hydrogen bonding in CD3CN, a stronger hydrogen bond acceptor. The acidity of the NH protons was confirmed when exchange with deuterium was observed in CD3OD (Figure 2). The coupling constants of the CHPh-CHPh protons (H5 and H10), at 11-12 Hz, are diagnostic of three-bond couplings between diaxial, vicinal protons.38-40 This feature indicates that the phenyl groups are equatorial in the five-membered Rh-N-C-C-N ring, in agreement with the X-ray structure (see below). Complex S,S-2c was characterized by X-ray crystal analysis (Figure 3). Table 1 compares its structure to those of 2a,5 the related ruthenium complex, [(p-cymene)RuCl(TsNCHPhCHPhNH2)],3 and bromide and iodide analogues described below. The amine hydrogen atoms were located in a difference map and refined isotropically, thereby demonstrating the presence of the intramolecular NH · · · Cl hydrogen bond. Further discussion of the structure is postponed until the other halide complexes have been described. It has recently been suggested that [(arene)Ru(monotosylamidoamine)] complexes form dimers in solution through hydrogen bonding between the NH protons of one molecule

Figure 3. Molecular structure of S,S-2c (thermal ellipsoids shown at 50% probability). Hydrogen atoms are omitted except for those of the amino group. The dotted line shows the hydrogen bond. Table 1. Selected Bond Lengths (Å) and Angles (deg) for 2a, 2c, 6a, 7a, and Ru complex a

bond M-X M-N(1) MsN(2) ∆r(M-N)d N(1)-M-N(2) N(1)-M-X N(2)-M-X X · · · N(1) X · · · HN(1) X · · · H-N(1)

2a (X ) Cl)

2c (X ) Cl)

6a (X ) Br)

7a (X ) I)

Rub (X ) Cl)

2.421(5) 2.103(6) 2.184(6) 0.081(8) 77.5(5) 84.1(2) 94.0(2) 3.032(7)

2.4175(5) 2.1069(17) 2.1649(17) 0.058(2) 78.70(6) 83.36(5) 91.81(5) 3.0183(19) 2.55(2) 111.7(17)

2.4968(7) 2.101(4) 2.186(4) 0.085(6) 77.27(15) 84.52(12) 94.65(10) 3.106(4) 2.61c 123c

2.7107(3) 2.122(3) 2.168(2) 0.046(4) 78.76(10) 84.01(8) 93.75(6) 3.263(3) 2.78(3) 115(2)

2.435(4) 2.116(9) 2.145(8) 0.029(12) 79.5(3) 81.1(2) 86.4(2) 2.968(10) 2.57c 104c

a Ref 5, H not located. b Ref 3 Ru ) [(p-cymene)RuCl(TsNCHPhCHPhNH2)]. c No esd available since hydrogen location could not be refined, or was not refined. d ∆r(M-N) is the difference between the two Rh-N distances.

and a sulfonyl oxygen of another, or between the NH protons of one molecule and the chloride of a second.41 This has been demonstrated by PGSE NMR experiments and calculations, and also by ESI mass spectrometry. An ESI mass spectrum of 2a (in CH3OH) showed a peak at m/z ) 603 for the monomer [M - Cl]+ and also at m/z ) 1241 for [M2 - Cl]+, suggesting that both monomer and dimer may be present in solution. We also wished to determine 15N NMR data for 2a as a basis for further study of catalytic intermediates. [15N]-TsDPEN was synthesized as a mixture of R,R and S,S isomers according to the method of Corey et al. (Scheme 2) with ca. 40% 15N.42 It was not necessary to resolve the racemic mixture because the 1 H NMR spectra of the associated metal complexes are identical. The DPEN ligand was tosylated to give 15N-labeled TsDPEN (1a-15N) in 46% yield. The FAB mass spectrum for 1a-15N

Figure 2. 1H NMR spectra of 2a in CD2Cl2 (top), CD3CN (middle), and CD3OD (bottom).

(35) Watanabe, M.; Ikagawa, A.; Wang, H.; Murata, K.; Ikariya, T. J. Am. Chem. Soc. 2004, 126, 11148. (36) Koike, T.; Ikariya, T. Organometallics 2005, 24, 724. (37) Soleimannejad, J.; Sisson, A.; White, C. Inorg. Chim. Acta 2003, 352, 121. (38) Tormena, C. F.; Vilcachagua, J. D.; Karcher, V.; Rittner, R.; Contreras, R. H. Magn. Reson. Chem. 2007, 45, 590. (39) Sudmeier, J. L.; Blackmer, G. L. Inorg. Chem. 1971, 10, 2010. (40) Pouzard, G.; Rajzmann, M.; Bodot, H.; Pujol, L. Org. Magn. Reson. 1973, 5, 209. (41) Pelagatti, P.; Carcelli, M.; Calbiani, F.; Cassi, C.; Elviri, L.; Pelizzi, C.; Rizzotti, U.; Rogolino, D. Organometallics 2005, 24, 5836.


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Scheme 2. Synthesis of [15N]-DPENa

a 40% 15N-labeled NH OAc was used, and a full range of isotopomers 4 were formed; only the S,S isomer is shown but a mixture of S,S and R,R isomers was formed.

Figure 5. 13C-13C COSY (125 MHz, CD2Cl2, 300 K) experiment, showing couplings between the terminal CO and acyl CO in the two diastereomers of 13C-labeled 3a. 103

Rh NMR chemical shifts were also recorded via a H-103Rh HMQC method making use of the coupling to the C5Me5 protons (data in Experimental Section). ReactivityofCp*RhCl(TsNCHPhCHPhNH2),2a-2c.1. Reactions with Carbon Monoxide, tert-Butylisonitrile, and Formic Acid. Rhodium complexes are known to catalyze both the decomposition of HCO2H43 and the water-gas shift reaction,44 thereby providing a potential source of CO. The reactivity of the catalytic species toward CO is therefore of interest when using HCO2H for transfer hydrogenation. CO (3 atm) was admitted to a solution of S,S-2a in CD2Cl2, and the reaction was followed by 1H NMR spectroscopy. Over a period of 5 days, the Cp* resonance of S,S-2a at δ 1.80 decreased in intensity and was replaced by two new Cp* proton signals at δ 1.68 and 1.83 with approximately equal intensities, indicating complete conversion to products. Over an extended period, the system evolves further and Cp*Rh(CO)2 is formed as the major product, as indicated by the associated 1H and 13C NMR spectra and IR spectrum. Analysis of the first stage of reaction suggested that the initial products were two diastereoisomers S,S,RRh and S,S,SRh of [Cp*Rh(CO){C(O)N(Ts)CHPhCHPhNH2}][Cl] (3a) with the same ligand chirality, but with opposite chirality at Rh. The 13C NMR spectrum of the two diastereomers of 3a shows resonances at δ 181.6 (d, JRhC ) 27 Hz) and 181.7 (d, JRhC ) 27 Hz) assigned to OCN and signals at δ 188.1 (d, JRhC ) 77 Hz) and 188.2 (d, JRhC ) 77 Hz) for the CO ligands. A 13 C-13C COSY spectrum of 3a (synthesized by reaction of 2a with 1 atm 13CO and 2 atm 12CO) showed couplings between RhCO at 188.1 and the OCN at δ 181.6, along with couplings between the CO at δ 188.2 and the OCN at δ 181.7 (Figure 5), consistent with the presence of two diastereomers. The 1H NMR spectrum (500 MHz, CD2Cl2, 300 K) of 3a showed two peaks for Cp* (δ 1.68 and 1.83), two for the tosyl methyl group (δ 2.28 and 2.31), for the CH protons of the backbone, and for the NH protons. All these observations are consistent with the formation of a pair of diastereoisomers. Complex 3a may form via substitution of chloride by CO, followed by migration of the amide bond of the chelating ligand onto CO, which creates a coordinatively unsaturated, planar, 16-electron, carbamoyl complex. CO could then bind from the front or the back, forming two diastereomers of 3a with the same ligand chirality, but with opposite chirality at Rh (Scheme 1

Figure 4. 1H-15N HMQC spectrum of 2a-15N (CD2Cl2, 300 K) showing couplings between the two nitrogen nuclei and the C5Me5 protons. δ(1H) horizontal, δ(15N) vertical.

compared well with the calculated spectrum. 15N-Labeled 2a (designated 2a-15N) was synthesized from 1a-15N, as described above. The 1H NMR spectrum (500 MHz, CD2Cl2, 300K) of 2a-15N displays coupling of both NH protons to 15N. In the 15N-enriched complex, Ha (numbering in Figure 2) generates two apparent triplet resonances with JNH ) 82 Hz on either side of the peak of the 14N isotopomer. Proton Hb generates two doublets with JNH ) 72 Hz on either side of the doublet of the 14N isotopomer. A 1H-15N HMQC spectrum showed a multiplet at δ -263.6 assigned as the amino nitrogen with coupling to both NH protons, H5, H10, and the C5(CH3)5 protons. A second multiplet at δ -210.6 with coupling to H5, H10, and the C5(CH3)5 protons was assigned as the amido nitrogen. A 1H–15N HMQC spectrum (50.7 MHz, CD2Cl2, 300 K) was recorded and showed Rh coupled doublets at δ -263.6 and -210.6 with JRhN ) 14 and 18 Hz, respectively (Figure 4). The observation of coupling between the TsDPEN ligand nitrogens and the protons of the Cp* ligand led us to try recording natural abundance 1H/15N HMQC spectra of complexes 2a-2c. This was found to be surprisingly straightforward. Complex 2b had peaks at δ -263.7 and -209.5 with JRhN ) 17 and 20 Hz, respectively, while complex 2c showed 15N peaks at δ -263.8 and -210.2 with JRhN ) 16 and 21 Hz, respectively. (42) Corey, E. J.; Imwinkelried, R.; Pikul, S.; Xiang, Y. B. J. Am. Chem. Soc. 1989, 111, 5493.

(43) King, R. B.; Bhattachayya, N. K. Inorg. Chim. Acta 1995, 237, 65. (44) Cheng, C. H.; Hendriksen, D. E.; Eisenberg, R. J. Am. Chem. Soc. 1977, 99, 2791.



Scheme 3. Proposed Mechanism for the Reaction of 2a with CO to Form 5a

3). Both ESI and FAB mass spectrometry revealed peaks at m/z ) 603 for [M - 2CO]+; the parent molecular ion was not detected. Insertion of CO into mononuclear amido and amidinato complexes has been observed previously to form carbamoyl complexes.45-48 Recently Rauchfuss et al. have reported a similar loss of chirality in related complexes, [Cp*Ir(L)(TsNCHPhCHPhNH2]+ (L ) PPh3, P(4-FC6H4)3, P(4MeOC6H4)3, or CO).22 On release of the pressure of CO from the NMR tube (before complete conversion to Cp*Rh(CO)2), 3a converted back to 2a sufficiently slowly to allow IR spectra to be recorded. The IR spectrum showed a peak at 2056 cm-1 consistent with a terminal CO ligand and a peak at 1600 cm-1 assigned to the acyl group of the bidentate ligand for 3a. When 13C-enriched CO was used, the peaks for the 13C isotopomers were observed at 2003 cm-1 (2012 cm-1 calcd, based on the change in reduced mass) and 1562 cm-1 (1566 cm-1 calcd). The reaction between 2a and CO may be accelerated by the presence of methanol. When complex 2a was dissolved in CD3OD with 3 atm of CO, the color changed immediately from orange to yellow. The NMR spectrum of the product solution was consistent with conversion to the two diastereoisomers of 3a, as described for the reaction in CD2Cl2. The 1H NMR spectrum (500 MHz, CD3OD, 300 K) of 3a showed two peaks for Cp* (δ 1.68 and 1.82), two for the tosyl methyl group (δ 2.25 and 2.31), a multiplet consistent with the CH protons of the ligand backbone at δ 4.48-4.52, and another at δ 5.03-5.08. Notably, the two isomers were formed initially with a 4:1 ratio (δ 1.68 dominant), but equalized in intensity after about an hour. This experiment therefore also suggests reversible CO coordination leading to thermodynamic equilibrium. Recently, Ikariya et al. showed that the related cyclometalated iridium chloride complexes react with AgBF4 and CO to form metal carbonyl cations with νCO very close to the values that we report for the terminal CO.49 Rauchfuss and co-workers have reported the complexes [Cp*Ir(CO)(TsCH2CH2NH2)][BF4], [Cp*Ir(CO)(Ts-1,2-C6H10NH2)][BF4], and [Cp*Ir(CO)(TsCHPhCHPhNH2)][BF4] with νCO of 2059, 2059, and 2064 cm-1.21,22 Reaction of 2a with tBuNC in CH3OH gives an immediate color change to yellow; extraction into toluene followed by (45) Piraino, P.; Bruno, G.; Tresoldi, G.; Farone, G.; Bombieri, G. J. Chem. Soc., Dalton. Trans. 1983, 2391. (46) Bryndza, H. E.; Fultz, W. C.; Tam, W. Organometallics 1985, 4, 939. (47) Joslin, F. L.; Johnson, M. P.; Mague, J. T.; Roundhill, D. M. Organometallics 1991, 10, 2781. (48) Rahim, M.; Ahmed, K. J. Organometallics 1994, 13, 1751.

Scheme 4. Reactions of 2a with tBuNC, HCO2H, LiBr, and KI

addition of KPF6 yields [Cp*Rh(CNtBu)(TsNCHPhCHPhNH2)][PF6] (4a) as a yellow powder (Scheme 4). The complex was crystallized from THF/hexane to give 4a as yellow needles. The IR spectrum shows νCN at 2112 cm-1 (KBr disk). The 13C NMR spectrum shows a resonance for tBuNC at δ 134.8 as a doublet of multiplets with JRhC ) 69 Hz. These data are consistent with those for Cp*RhCl2(CNMe), which exhibits a doublet of triplets at δ 136.3 with JRhC ) 69 Hz and JNC ) 20 Hz.50 The reaction of 2a with HCO2H (10 equiv) in methanol was performed to establish how 2a reacts with a single component of the system for catalytic transfer hydrogenation. Upon addition of formic acid, the color of the solution turned immediately from orange to red, and a red powder was isolated by adding 2 equiv of NaBPh4 to a solution of the metal complex in methanol. ESI mass spectrometry (M+ ) 557) was consistent with formation of the triply bridged dimer [{Cp*Rh}2(µ-H)(µ-Cl)(µ-HCO2)][BPh4] ([5a][BPh4]), and this assignment was con(49) Arita, S.; Koike, T.; Kayaki, Y.; Ikariya, T. Organometallics 2008, 27, 2795. (50) Jones, W. D.; Duttweiler, R. P.; Feher, F. J. Inorg. Chem. 1990, 29, 1505.


Blacker et al.

Figure 6. Molecular structure of the cation in [5a][BPh4] · OEt2 (thermal ellipsoids shown at 50% probability). Hydrogen atoms are omitted except for hydride. Selected bond lengths [Å] and angles [deg]: Rh(1) · · · Rh(2) 2.8251(3), Rh(1)-O(2) 2.1044(18), Rh(2)-O(1) 2.117(2), Rh(1)-H(1) 1.73(3), Rh(2)-H(1) 1.77(3), Rh(1)-Cl(1) 2.4290(7), Rh(2)-Cl(1) 2.4201(7); Rh(1)-Cl(1)-Rh(2) 71.27(2), Rh(1)-O(2)-C(21) 123.79(18), Rh(2)-O(1)-C(21) 123.25(18), O(1)-C(21)-O(2) 128.3(3).

firmed by X-ray analysis using a crystal grown from acetone/ Et2O at -20 °C (Scheme 4). Related triply bridged cationic dimers of general formula [{Cp*Rh}2(µ-H)(µ-OCOR)2][PF6] have been reported by Maitlis et al.51 The crystal structure of [5a][BPh4] · OEt2 (Figure 6) reveals Rh(1)-O(2) and Rh(2)-O(1) distances of 2.1044(18) and 2.117(2) Å, respectively, longer than those in [{Cp*Rh}2(µ-H)2(µ-CO2CH3)]+ (2.071 and 2.070 Å).52 The hydride in the cation 5a was readily located on a difference map. Rh(1)-H(1) and Rh(2)-H(1) distances of 1.73(3) and 1.77(3) Å, respectively, are comparable to those in [{Cp*Rh}2(µ-H)2(µ-CO2CH3)]+ (1.73 Å). The geometry of the bridge is different from that of its analogues. The Rh · · · Rh distance for the cation 5a of 2.8251(3) Å may be compared with 2.680 Å in [{Cp*Rh}2(µ-H)2(µ-CO2CH3)]+.52 The average Rh(1)-Cl distance of 2.425(1) Å is shorter than that in [{Cp*Rh}2(µ-Cl)3]+ (2.457 Å),53 while the Rh(1)-Cl(1)-Rh(2) angle of 71.27(2)° for 5a is much more acute than that for [{Cp*Rh}2(µ-Cl)3]+ (81.23° average). The O(1)-C(21)-O(2) angle of 128.3(3)° is wider than that of [{Cp*Rh}2(µ-H)2(µ-CO2CH3)]+ (124.80°). The 1H NMR spectrum (500 MHz, (CD3)2CO, 300 K) of [5a][BPh4] showed a triplet at δ -8.67 (JRhH ) 27 Hz) for the bridging hydride, a singlet at δ 1.83 for the Cp* protons, and a triplet at δ 7.41 (JRhH ) 4 Hz) for the bridging formate proton. The isolation of [5a][BPh4] raises the question of whether cation 5a could be formed under catalytic conditions when the formic acid is buffered by triethylamine. As an experimental test, we investigated the reduction of d6-acetone by d15triethylamine/formic acid catalyzed by 2a in CD3CN solution. The molar ratio of acetone substrate to catalyst was 20:1. The reaction was carried out both at 250 K and at 300 K with the same result. Examination of the hydride region of the 1H NMR spectrum showed one hydride species only with a triplet at δ -8.76 (J ) 27 Hz), consistent with significant formation of the hydride-bridged cation 5a. The same species was observed with CD3OD and CD2Cl2 as solvents. The surprising observation of 5a under catalytic conditions raises the question of whether reaction of the catalyst with (51) Cook, J.; Hamlin, J. E.; Nutton, A.; Maitlis, P. M. J. Chem. Soc., Dalton. Trans. 1981, 2342. (52) Nutton, A.; Bailey, P. M.; Maitlis, P. M. J. Organomet. Chem. 1981, 213, 313. (53) Gauthier, S.; Quebatte, L.; Scopelliti, R.; Severin, K. Inorg. Chem. Commun. 2004, 7, 708.

Figure 7. 1H NMR (CHPh and NH2 region) spectra of 2a (top), 6a (middle), and 7a (bottom) 500 MHz, CD3CN, 300 K.

formic acid is a significant deactivation pathway. We compared the catalytic activities of 2a and [5a][BPh4] for the transfer hydrogenation of the imine 6,7-dimethoxy-1-methyl-3,4-dihydroisoquinoline. Under conditions when 2a gave excellent activity and ee’s of 90%,20,54 complex [5a][BPh4] showed very poor activity. However, the catalytic system of {5a + 1a} gave activity and selectivity comparable to that of 2a. Thus we conclude that reaction of 2a with formic acid to form 5a may occur to some extent under catalytic conditions despite the presence of NEt3. Nevertheless, catalytic activity is not necessarily blocked because the presence of free chiral ligand may allow recovery of activity. 2. Reactions with Halide Salts. The addition of LiBr or KI to a solution of S,S-2a in methanol resulted in quantitative conversion to the corresponding bromide Cp*RhBr(S,STsNCHPhCHPhNH2) (S,S-6a) or iodide Cp*RhI(S,S-TsNCHPhCHPhNH2) (S,S-7a), respectively (Scheme 4). The addition of iodide salts is accompanied by a color change from pale orange to dark red. Both complexes were characterized by 1H and 13C NMR spectroscopy, FAB mass spectrometry, and X-ray crystallography. The 1H NMR spectra show most variation between 2a, 6a, and 7a in the proton chemical shifts of the ligand backbone and NH2 groups, as illustrated in Figure 7. Complex S,S-6a was recrystallized by slow diffusion of hexane into a concentrated solution in THF. Crystals of S,S7a · CH3OH were obtained by slow evaporation of a solution in CH3OH. Selected bond lengths, angles, and interatomic distances are compared to those for 2c and literature values for 2a and for Ru ((p-cymene)RuCl(TsNCHPhCHPhNH2)) in Table 1. The (54) Grace, J. The Rh catalysed asymmetric transfer hydrogenation of Imines; a mechanistic study. Ph.D. Thesis, University of York, 2007.



Figure 9. Molecular structure and unit cell for [8c-H2O][OTf] (thermal ellipsoids shown at 50% probability). Hydrogen atoms and anions are omitted for clarity. Selected bond lengths [Å] and angles [deg]: Rh(1)-N(1) 2.1328(19), Rh(1)-O(3) 2.1631(18), Rh(1)-N(2) 2.141(2); N(1)-Rh(1)-N(2) 77.61(8), N(1)-Rh(1)-O(3) 85.24(7), N(2)-Rh(1)-O(3) 90.08(7).

Figure 8. Molecular structures of S,S-6a (above) and S,S7a · CH3OH (below) (thermal ellipsoids shown at 50% probability). Hydrogen atoms are omitted except for the amino group and methanol OH. The hydrogen bonds are shown as dotted lines.

amine hydrogen atoms were located by difference maps; their positions were refined successfully for 7a but their positions were constrained in the refinement of 6a. The structures of 2a, 2c, 6a, and 7a are very similar and also close to that of Ru.3 The most important features are (a) that the Rh-N(1) (NH2) bond is significantly shorter than the Rh-N(2) (NS) bond and (b) that each complex possesses an intramolecular NH · · · X hydrogen bond. The phenyl groups lie in equatorial positions on the five-membered Rh-N-C-C-N ring, as in related structures and in agreement with the evidence provided by the 1 H NMR data (see above).22 The Rh-NH2 bond length is invariably shorter than the Rh-NS bond length, but the differences ∆r(M-N) are larger for 2a and 6a (0.081(8) and 0.085(6) Å) than for 2c and 7a (0.058(2) and 0.046(4) Å). The changes arise from an increased Rh-N(1) and decreased Rh-N(2) distance in 7a and a decreased Rh-N(2) distance in 2c. The esd on ∆r(M-N) for the ruthenium complex is too large to compare to the values for the other complexes with confidence. The N · · · X and NH · · · X distances are consistent with an NH · · · X hydrogen bond in each complex (X ) Cl, Br, I). The NH · · · X distances are shorter than the sum of the van der Waals radii by 0.40, 0.44, and 0.40 Å in 2c, 6a, and 7a, respectively (van der Waals radii of Cl 1.75, Br 1.85, I 1.98, and H 1.20 Å).55,56 Complex 7a crystallizes as the methanol solvate and exhibits a further hydrogen bond between the methanol oxygen and the NH hydrogen that does not interact with iodine (Figure 8). In 2c, there are also short SdO(1) · · · HC(30) (55) Rowland, R. S.; Taylor, R. J. Phys. Chem. 1996, 100, 7384. (56) Bondi, A. J. Phys. Chem. 1964, 68, 441.

and SdO(1) · · · HC(12) contacts with O · · · C of 2.881(3) and 2.899(2) Å, respectively. The intramolecular hydrogen bonds have been noted in previous structures, but the striking difference in the Rh-N distances has not been emphasized, perhaps because the esd’s on the older structures were larger than on those reported here.3,5,15 3. Halide Abstraction. In an attempt to provide a route to a crystalline analogue of the CO complex 3a, the reaction of S,S-2c with AgOTf (OTf ) CF3SO3) in CH3OH was carried out. The chloride abstraction succeeded although the CO reaction proved unsuccessful; the addition of AgOTf to a solution of 2c in methanol led to an immediate color change from orange to magenta and precipitation of AgCl. After filtering off the insoluble salt and removing the solvent under reduced pressure, a purple solid was obtained. This solid was identified as [Cp*Rh(4-FC6H4SO2NCHPhCHPhNH2)][OTf] ([8c][OTf]). A sample was dissolved in CD3CN for NMR analysis, resulting in a yellow solution, suggesting CD3CN coordination to Rh. Attempts to locate the coordinated CH3CN by running a 1H NMR spectrum in CH3CN using solvent suppression were unsuccessful, although exchange could be occurring on the NMR time scale. The coordination of solvent was supported by an IR band at 2239 cm-1 consistent with the CN stretch of coordinated CH3CN.57 Complex [8c][OTf] was characterized by 1H, 13C, and 19F NMR spectroscopy in CD3CN. The 1H NMR spectrum displayed peaks at δ 4.50 and 3.95 for the NH2 protons, a resonance at δ 1.83 for Cp*, and signals at δ 4.15 and δ 3.76 for the CHPh protons. The 13C NMR resonances of the sulfonyl aromatic ring carbons all show couplings to 19F, as for 2c. A diffuse reflectance UV-vis spectrum of [8c][OTf] showed λmax ) 520 nm. The ESI mass spectrum in CH3OH showed the expected peak at m/z ) 607 for [M]+, without coordinated solvent. A crystal suitable for X-ray analysis was grown by vapor diffusion of hexane into a solution of [8c][OTf] in CH3OH/ toluene (1:1). The structure obtained was a derivative of [8c][OTf] with a water molecule coordinated to the rhodium center, [8c-H2O][OTf] (Figure 9). The most significant difference in the structure of [8c-H2O][OTf] from those of 2a and 2c is a change from a chiral to an achiral space group. Complex [8c-H2O][OTf] crystallized in the achiral space group P1j, whereas those used to solve structures of 2a-2c were the chiral space groups P21 or P212121. The hydrogen atoms of the coordinated water and amine were located in a difference map (57) Rigby, W.; Bailey, P. M.; McCleverty, J. A.; Maitlis, P. M. J. Chem. Soc., Dalton. Trans. 1979, 371.


and refined isotropically. Figure 9 shows that the two molecules in the cell have opposite chirality at both Rh and ligand, and it appears that the ligand has racemized. At the molecular level, there are also important changes: the Rh-NH2 distance in [8cH2O][OTf] is 0.026(3) Å longer than in 2c, while the Rh-NTs distance is 0.024(3) Å shorter, with the result that ∆r(Rh-N) is insignificant. The Rh-O bond in [8c-H2O][OTf] of 2.1631(18) Å is comparable to the mean value in [Cp*Rh(OH2)3][OTf]2 (2.156 Å)58 and that in [Cp*Rh(PMe3)(OH2)(C6F5CF2)][BF4] (2.164(7) Å).59 The amine hydrogen atoms engage in hydrogen bonding to the oxygen atoms of the triflate counterion, while the water hydrogen atoms form hydrogen bonds with the fluorine and oxygen atoms of the counterion and with the sulfonyl oxygens. The crystal structure leaves the question open whether the majority of the complex has racemized or whether the result depends on the selection of crystals. Further evidence for racemization of the TsDPEN ligand was obtained by measuring the optical rotation of a solution of S,S-2c in CHCl3, reacting this with AgOTf to form [8c][OTf], and measuring the optical rotation of the resulting solution at constant concentration. The solution of complex S,S-2c had an optical rotation of -1.5°, whereas the solution of the product [8c-H2O][OTf] had an optical rotation of -0.1°. Furthermore, subsequent conversion of [8c-H2O][OTf] back to 2c (via addition of Bu4NCl) resulted in an optical rotation of -0.6°, well short of the initial optical rotation. This significant reduction of optical activity for the product solution is evidence that a substantial proportion of the ligand in the complex has racemized and the effect is not confined to a few crystals. A second crop of crystals of [8c][OTf] was grown as described above, but under anhydrous conditions, and proved to have a different structure. In the absence of water or a suitable coordinating solvent, the cationic 16-electron complex dimerized. Although the structure is of poor quality, the principal features are important. An oxygen of the sulfonyl group of one molecule has filled the vacant site at that of the other and Vice Versa to give the dimer [Cp*Rh(4-FC6H4SO2NCHPhCHPhNH2)]2[OTf]2 ([8c]2[OTf]2, structure below). As with [8cH2O][OTf], the structure of [8c]2[OTf]2 was solved with the achiral P1j space group and opposite enantiomers are paired in the unit cell. For comparison, [Cp*Rh(R1COCHCOR2)][PF6] has been shown to exist as a dimer by coordination of the central acetylacetonate carbon of one complex to Rh in another and Vice Versa.60 An ESI mass spectrum of [8c][OTf] showed a significant peak at m/z ) 1363 for [M2(SO3CF3)]+, providing some evidence for the existence of the dimer in solution as well as in the solid state.

The racemization of the ligand in [8c-H2O][OTf] is of importance in asymmetric transfer hydrogenation. A possible mechanism for this racemization of the ligand is outlined (Scheme 5): step 1 is the reversible dissociation of coordinated

Blacker et al. Scheme 5. Possible Mechanism for the Racemization of the FsNCHPhCHPhNH2 Ligand in [8c-S]+ (S, ) solvent, Fs ) 4-FC6H4SO2)

solvent; step 2 is a β-elimination; step 3 is an imine-enamine tautomerism, which scrambles the chirality; steps 4 and 5 are the reverse of steps 1 and 2, so both enantiomers are as likely to be formed and a racemic mixture ensues. The meso isomer is likely to be much higher in energy and disfavored. This mechanism remains unproven, but it provides a plausible explanation of the observations. The reactivity of [8c][OTf] toward two-electron donors was studied as a route to new cationic Rh complexes; although the reaction with CO was unsuccessful, an adduct was formed by reaction with tBuNC. Upon addition of 1 equiv of tBuNC to a solution of 8c in CH3OH, an immediate color change from magenta to yellow was observed. Workup by removing the solvent under reduced pressure, extraction into CH3CN, and removal of CH3CN under vacuum gave a yellow solid, which has been characterized by 1H, 13C, and 19F NMR spectroscopy and ESI mass spectrometry and was found to be [Cp*Rh(CNtBu)(4-FC6H4SO2NCHPhCHPhNH2)][OTf] (4c).

Conclusions A group of 18-electron half-sandwich rhodium complexes, of the type [Cp*Rh(X)(4-RC6H4SO2NCHPhCHPhNH2)] (X ) Cl, Br, I; R ) Me, tBu, F), of significance in asymmetric transfer hydrogenation catalysis5-7,10,15,17,20 has been isolated. The complexes have been characterized by multinuclear NMR spectroscopy and by X-ray crystallography. The NMR spectra show significant solvent dependence of the 1H chemical shift of the NH2 protons characteristic of NH · · · X hydrogen bonding. The 1H–15N HMQC NMR spectra show that the chemical shifts of the inequivalent nitrogen atoms are separated by ca. 50 ppm; moreover, the resonances exhibit significant coupling to the C5Me5 protons and to the rhodium nucleus. These methods offer scope for wider application for characterization of the nitrogen environment in solution. Notable features of the X-ray structure are the significantly longer Rh-N bonds to the amido nitrogen than the amino nitrogen and the presence of an intramolecular



Table 2. Crystallographic Data for 2c, [5a][BPh4] · OEt2, 6a, 7a, and [8c · H2O][OTf] formula M a/Å b/Å c/Å R/deg β/deg γ/deg V/Å3 T/K space group Z µ(Mo KR)/mm-1 reflns measd reflns indep Rint final R[I > 2σ(I)] final R (all data) CCDC number

2c

[5a][BPh4] · OEt2

6a

7a

[8c-H2O][OTf]

C30H33ClFN2O2RhS 643.00 7.8845(4) 11.3176(5) 31.7839(14)

C49H62BClO3Rh2 951.07 11.7528(10) 13.6373(12) 14.2877(12) 106.264(2) 92.571(2) 92.506(2) 2192.3(3) 110(2) P1j 2 0.854 22 323 10 753 0.0221 R1 ) 0.0351 wR2 ) 0.0847 R1 ) 0.0440 wR2 ) 0.0889 703216

C31H36BrN2O2RhS 683.50 8.3927(4) 16.1471(8) 21.6575(11)

C32H40IN2O3RhS 762.53 8.4842(5) 9.1856(6) 20.6402(13)

C34.5H38.5F4N2O6RhS2 820.20 12.286(5) 12.454(5) 13.377(5) 107.266(7) 113.603(7) 97.270(7) 1719.2(12) 110(2) P1j 2 0.688 23 383 8538 0.0267 R1 ) 0.0336 wR2 ) 0.0848 R1 ) 0.0380 wR2 ) 0.0869 703217

2836.2(2) 100(2) P212121 4 0.807 42 801 8180 0.0289 R1 ) 0.0254 wR2 ) 0.0625 R1 ) 0.0268 wR2 ) 0.0672 703218

NH · · · X hydrogen bond that may prepare the complex for proton abstraction.3,5,15 Direct reaction of Cp*Rh(Cl)(S,S-TsNCHPhCHPhNH2) with CO results in the reversible formation of [Cp*Rh(CO){C(O)N(Ts)CHPhCHPhNH2}][Cl] as two diastereomers, S,S,RRh and S,S,SRh, with different configurations at rhodium. This species has been identified by 2D-13C-13C NMR spectroscopy with 13CO labeling and by IR spectroscopy. The growth of the second isomer may be monitored during reaction in d4-methanol as solvent. Further reaction with CO leads to the complete displacement of the nitrogen ligands. The nitrogen ligands are also completely displaced by reaction of 2a with formic acid, yielding dinuclear [{Cp*Rh}2(µ-H)(µ-Cl)(µ-HCO2)][BPh4], [5a][BPh4], closely related to carboxylate complexes observed previously.51 Cation 5a is also detected under catalytic conditions for reduction of d6-acetone with d15-NEt3 and formic acid; that is, the buffering effect of triethylamine is not sufficient to prevent some reaction. Although formation of complex [5a][BPh4] represents a deactivation path for the catalyst, catalytic activity can be recovered by reaction of [5a][BPh4] with the ligand 1a. Reaction of Cp*RhCl(S,S-4-FC6H4SO2NCHPhCHPhNH2) with AgOTf generates a purple 16-electron complex that exists as the dimer [Cp*Rh(4-FC6H4SO2NCHPhCHPhNH2)]2[OTf]2 in the crystalline state. In the presence of adventitious water, it is isolated as a salt of the aqua cation [Cp*Rh(4-FC6H4SO2NCHPhCHPhNH2)(OH2)][OTf]. The notable feature of this reaction is that it results in racemization of both CHPh centers (i.e., formation of a mixture of R,R and S,S from initial S,S material). Thus halide abstraction in this manner would result in loss of enantioselectivity of the catalyst.

Experimental Section General Procedure. Unless otherwise stated, synthetic work was carried out in air with untreated solvents. Commercially available reagents were obtained from the following sources: [Cp*RhCl2]2, R,R and S,S-DPEN, R,R and S,S-TsDPEN, 6,7-dimethoxy-3,4dihydroisoquinoline (donated by Piramal Healthcare); RhCl3 · xH2O (Precious Metals Online); hexamethyl(dewar)benzene, benzil, 4-flu(58) Eisen, M. S.; Haskel, A.; Chen, H.; Olmstead, M. M.; Smith, D. P.; Maestre, M. F.; Fish, R. H. Organometallics 1995, 14, 2806. (59) Hughes, R. P.; Lindner, D. C.; Rheingold, A. L.; Liable-Sands, L. M. J. Am. Chem. Soc. 1997, 119, 11544. (60) Rigby, W.; Lee, H. B.; Bailey, P. M.; McCleverty, J. A.; Maitlis, P. M. J. Chem. Soc., Dalton. Trans. 1979, 387.

97.4630(10) 2935.0(3) 110(2) P212121 4 2.045 32 890 8364 0.0803 R1 ) 0.0429 wR2 ) 0.1177 R1 ) 0.0548 wR2 ) 0.1212 703215

1594.92(17) 110(2) P21 2 1.603 21 931 7863 0.0244 R1 ) 0.0248 wR2 ) 0.0585 R1 ) 0.0272 wR2 ) 0.0663 703214

orobenzenesulfonyl chloride, 4-tert-butylbenzenesulfonyl chloride, triethylamine, lithium, ammonium-15N acetate (98 atom % 15N), formic acid (Aldrich); tosyl chloride (Alfa Aesar); d15-triethylamine from Cambridge Isotope Laboratories. Ar, N2, CO, and H2 gases (BOC) and were used as received without further purification. [Cp*RhCl2]2 was also made as previously reported.61 When necessary, dry solvents (Fisher Scientific AR or HPLC grade) were prepared by refluxing over Mg/I2 (CH3OH), CaH2 (CH3CN, CH2Cl2), or sodium/benzophenone (THF) and were degassed and, along with air- and moisture-sensitive materials, stored under a dry Ar atmosphere. Manipulations of these compounds were performed using standard Schlenk, high-vacuum, and glovebox techniques. Deuterated solvents (Aldrich) were dried using CaH2 (CD3OH, CD3CN, and CD2Cl2) and degassed by three freeze-pump-thaw cycles. Unless otherwise stated, DPEN, TsDPEN, 4-FC6H4SO2DPEN, 4-tBuC6H4SO2DPEN, and all metal complexes have been synthesized and used as single diastereomers, with either the R,R or S,S configuration at the ligand, which, when coordinated to the metal, enforces the opposite chirality at Rh; the metal complex of the S,S ligand will have the R configuration at the metal center and Vice Versa. When referring to DPEN and related ligands, the specific chirality is not always mentioned, as both have been used interchangeably. Most NMR spectra were recorded on Bruker AMX500, DRX400, or AMX300 spectrometers. The chemical shift values are reported relative to the residual protons of the deuterated solvent, according to the standard Bruker list. Numbering of hydrogen and carbon atoms refers to Figure 2. 1H-103Rh HMQC spectra were run on a Bruker Avance II 700 MHz spectrometer calibrated against [Cp*RhCl2]2 at δ 2303.62 1H-15N HMQC spectra were referenced to pyridine as δ 0. FAB mass spectra were recorded on a VG AutoSpec, and ESI mass spectra were recorded on a Thermo Electron LCQ Classic. Infrared spectra were recorded on a Unicam RS 10000E FTIR instrument. Polarimetry measurements were made in a Jasco DIP-370 digital polarimeter using a 2 mL cell and CHCl3 solvent. Microanalysis was carried out by Elemental Microanalysis Ltd., Okehampton, Devon. Synthesis of 4-RC6H4SO2DPEN Ligands.37 DPEN was dissolved in CH2Cl2 in a two-necked, 500 mL round-bottom flask. The solution was cooled to 0 °C, NEt3 was added, and the mixture was stirred magnetically. The appropriate sulfonyl chloride was (61) Kang, J. W.; Moseley, K.; Maitlis, P. M. J. Am. Chem. Soc. 1969, 91, 5970. (62) Tedesco, V.; von Philipsborn, W. Magn. Reson. Chem. 1996, 34, 373.

1444 Organometallics, Vol. 28, No. 5, 2009 dissolved in CH2Cl2 and added to a dropping funnel, which was placed in the side neck of the round-bottom flask. The sulfonyl chloride solution was added to the stirred DPEN/NEt3 solution dropwise over an hour, and the solution was warmed to room temperature and stirred overnight. It was washed successively with water (80 mL), NaHCO3 solution (80 mL), water (80 mL), and brine (80 mL) and then dried over MgSO4; the solvent was removed under reduced pressure to give a white solid, which was crystallized from EtOAc/hexane. 4-tBuC6H4SO2DPEN (1b). 1b was synthesized as described above from DPEN (2.10 g, 9.9 mmol) and NEt3 (2.10 mL, 14.9 mmol) in CH2Cl2 (150 mL) and 4-tert-butylbenzenesulfonyl chloride (2.00 g, 8.7 mmol) in CH2Cl2 (50 mL). Yield: 1.72 g (48%). 1H NMR (500 MHz, CD2Cl2, 300 K, numbering as in Figure 2): δ 1.29 (9H, s, (CH3)3C), 4.14 (1H, d, JHH ) 5 Hz, H5 or H10), 4.35 (1H, d, JHH ) 5 Hz, H5 or H10), 7.12-7.15 (10H, m, Ph), 7.21 (2H, d, JHH ) 8 Hz, H2), 7.33 (2H, d, JHH ) 8 Hz, H3). 13C NMR (125 MHz, CD2Cl2, 300 K): δ 31.36 (s, (CH3)3C), 35.36 (s, (CH3)3C), 61.0 (s, C5 or C10), 63.86 (s, C5 or C10), 126.10 (s, C3 or C4), 127.15 (s, C3 or C4), 127.17 (s, Ph), 127.60 (s, Ph), 127.81 (s, Ph), 128.70 (s, Ph), 128.83 (s, Ph), 129.99 (s, Ph), 137.60 (s, C1), 140.15 (s, C6 or C11), 142.30 (s, C6 or C11), 156.08 (s, C4). MS (FAB, m/z): 410 ([MH+]). IR (KBr/cm-1): 3356 (νNH), 3298 (νNH), 3291 (br, νNH). Anal. Calcd for C24H28N2O2S: C, 70.55; H, 6.91; N, 6.86. Found: C, 70.84; H, 7.08; N 6.87. 4-FC6H4SO2DPEN (1c). 1c was synthesized as described above from DPEN (2.46 g, 11.6 mmol) and NEt3 (2.40 mL, 17.4 mmol) in CH2Cl2 (150 mL) and 4-fluorobenzenesulfonyl chloride (2.00 g, 10.4 mmol) in CH2Cl2 (50 mL). Yield: 2.30 g (60%). 1H NMR (500 MHz, CD2Cl2, 300 K, numbering as in Figure 2): δ 4.17 (1H, d, JHH ) 5 Hz, H5 or H10), 4.38 (1H, d, JHH ) 5 Hz, H5 or H10), 6.86 (2 H, apparent t, JHH ) 9 Hz, JHF ) 9 Hz, H2), 7.17-7.20 (10H, m, Ph), 7.41 (2H, dd, JHH ) 9, JHF ) 5 Hz, H3). 13C NMR (125 MHz, CD2Cl2, 300 K): δ 60.86 (s, C5 or C10), 63.84 (s, C5 or C10), 116.18 (d, JFC ) 23 Hz, C2), 127.06 (s, Ph), 127.55 (s, Ph), 128.00 (s, Ph), 128.04 (s, Ph), 128.84 (s, Ph), 128.94 (s, Ph), 129.96 (d, JFC ) 9 Hz, C2), 136.91 (d, JFC ) 4 Hz, C4), 140.09 (s, C6 or C11), 142.19 (s, C6 or C11), 165.09 (d, JFC ) 251 Hz, C1). 19F NMR (480 MHz, CD2Cl2, 300 K): δ -107.54 (s). MS (FAB, m/z): 371 ([MH]+). IR (KBr/cm-1): 3350 (νNH), 3338 (νNH), 3291 (νNH). Anal. Calcd for C20H19FN2O2S: C, 64.85; H, 5.17; N, 7.56. Found: C, 64.72; H, 5.16; N, 7.49. 15 N-Labeled DPEN.42 A mixture of benzil (3.31 g, 15.8 mmol), cyclohexanone (1.63 mL,15.8 mmol), ammonium acetate (7.5 g 14 N and 5.24 g 15N), and acetic acid (30 mL) was refluxed for 1 h. The cooled mixture was slowly poured into vigorously stirred water (50 mL) and stirred for an additional 2 h, then left overnight. The crystals formed were collected by filtration, washed with water (4 × 10 mL), and dried in Vacuo to give 5-spirocyclohexyl-2,3diphenyisoimidazole (A) (4.41 g, 97%), which was used in the next step without further purification. To a magnetically stirred solution of A (4.41 g, 15.3 mmol) in THF (20 mL) and NH3 (25 mL) at -78 °C was added Li (601 mg, 85.9 mmol). Stirring was continued for 2 h and the reaction monitored by TLC, at which time all of the starting material had been consumed. Ammonium chloride (4.0 g) was added at -78 °C and the reaction mixture warmed very slowly to evaporate the NH3. When the temperature of the reaction mixture reached 0 °C, water (20 mL) and Et2O (20 mL) were added, the mixture was shaken, and the phases were separated. The aqueous phase was washed with Et2O (3 × 20 mL), and the combined organic extracts were washed with brine (20 mL), dried over Na2SO4, filtered, and concentrated in Vacuo to a volume of 15 mL. The resulting solution was cooled to 0 °C, 2 M HCl (15 mL) was added with vigorous stirring, and the solution was warmed to room temperature and stirred for 1 h. Water (30 mL) and CH2Cl2 (30 mL) were added, the mixture was shaken, and the phases were separated. The organic phase was washed with water (10 mL), and

Blacker et al. the combined aqueous phases were washed with CH2Cl2 (20 mL). The aqueous phase was treated with 2 M NaOH (20 mL) and extracted with CH2Cl2 (5 × 10 mL). The combined organic phases were dried over Na2SO4, filtered, and concentrated in Vacuo to give 15 N-labeled DPEN (1.36 g, 44%). 1H NMR (500 MHz, CDCl3, 300 K): δ 1.98 (4H, s), 4.09 (2H, s), 7.17-7.33 (10H, m). MS (FAB, m/z): 213 ([MH]+), 214 (singly labeled [MH]+), 215 (doubly labeled [MH]+). 15 N-Labeled TsDPEN (1a-15N). 1a-15N was synthesized as described above from 15N-labeled DPEN (178 mg, 0.84 mmol), NEt3 (150 µL, 1.1 mmol) in CH2Cl2 (30 mL), and tosyl chloride (122 mg, 0.64 mmol) in CH2Cl2 (15 mL). Yield: 109 mg (46%). 1 H NMR (500 MHz, CDCl3, 300 K): δ 2.33 (3H, s, CH3), 4.16 (1H, d, JHH ) 6 Hz, CHNH2), 4.39 (1H, d, JHH ) 6 Hz, CHNTs), 6.98 (2H, d, JHH ) 8 Hz, H4), 7.12-7.18 (10H, m, Ph) Ar, 7.32 (2H, d, JHH ) 8 Hz, H2). MS (FAB, m/z, relative intensity): 367 (63, [MH]+ 14N/14N/12C/32S isotopomer), 368 (100, [MH]+14 N/14N/13C/32S, 14N/15N/12C/32S isotopomers), 369 (69, [MH]+14 N/14N/12C/34S, 14N/15N/13C/32S, 15N/15N/12C/32S isotopomers), 370 (37, [MH]+ 14N/15N/12C/34S, 15N/15N/13C/32S isotopomers), 371 (18, [MH]+ 15N/15N/12C/34S isotopomer), 372 (6, [MH]+ 15N/15N/ 13 34 C/ S isotopomer). General Synthesis for Cp*RhCl(4-RC6H4SO2NCHPhCHPhNH2).17 [Cp*RhCl2]2 and 4-RC6H4SO2DPEN were dissolved in CH2Cl2, NEt3 was added dropwise with stirring, and the resulting solution was stirred for 1 h. The solution was washed with water, dried over MgSO4, and crystallized from CH2Cl2/hexane as orange needles. Cp*RhCl(TsNCHPhCHPhNH2) (2a). [Cp*RhCl2]2 (2.0 g, 3.2 mmol), TsDPEN (2.4 g, 6.5 mmol), and NEt3 (1.3 g, 12.9 mmol) were reacted in CH2Cl2 (120 mL), to give 2a (4.2 g, 93%). 1H NMR (500 MHz, CDCl3, 300 K, numbering as in Figure 2): δ 1.80 (15H, s, C5Me5), 2.21 (3H, s, Me), 3.46 (1H, br d, JHH ≈ 11 Hz, NHb), 3.70 (1H, ddd, JHH ) 3, 12, 14 Hz, H10), 3.84 (1H, br apparent t, JHH ≈ 12, 12 Hz, NHa), 3.95 (1H, d, JHH ) 11 Hz, H5), 6.64 (2H, d, JHH ) 7 Hz, H7 or H8), 6.76 (2H, t, JHH ) 7 Hz, H7 or H8), 6.80-6.85 (5H, m, H3, H9, and H12), 7.09-7.14 (3H, m, H13 and H14), 7.29 (2H, d, H2). 13C NMR (125 MHz, CD2Cl2, 300 K): δ 9.97 (s, C5Me5) 21.40 (s, Me), 69.91 (s, C5), 72.05 (s, C10), 94.92 (d, JRhC ) 8 Hz, C5Me5), 126.83 (s, C3, C9, or C13), 127.54 (s, C7 or C8), 127.59 (s, C3, C9, or C13), 128.27 (s, C3, C9, or C13), 128.49 (s, C2), 128.78 (s, C13 or C14), 128.97 (s, C13 or C14), 129.34 (s, C7 or C8), 139.79 (s, C1), 140.06 (s, C6 or C11), 140.29 (s, C6 or C11), 142.27 (s, C4). 15N NMR (50.7 MHz, CD2Cl2, 300 K): δ -263.6 (d, JRhN ) 14 Hz, NH2), -210.6 (d, JRhN ) 18 Hz, NTs). 103Rh-1H HMQC (22.18 MHz, CDCl3, 300 K): δ 2550.5. MS (FAB, m/z): 639 ([MH]+). IR (KBr/ cm-1): 3281 (νNH), 3209 (νNH). 1 H NMR (500 MHz, CD3CN, 300 K): δ 1.80 (15H, s, C5Me5), 2.24 (3H, s, Me), 3.60 (1H, br apparent t, JHH ≈ 12, 12 Hz, NHa), 3.78 (1H, ddd, JHH ) 3, 11, 14 Hz, H10), 3.91 (1H, d, JHH ) 11 Hz, H5), 4.41 (1H, br d, JHH ≈ 9 Hz, NHb), 6.68 (2H, d, JHH ) 8 Hz, H7 or H8), 6.77 (2H, t, JHH ) 7 Hz, H7 or H8), 6.83-6.88 (5H, m, H3, H9, and H12), 7.11-7.14 (3H, m, H13 and H14), 7.32 (2H, d JHH ) 8 Hz, H2). Cp*RhCl(4-tBuC6H4SO2NCHPhCHPhNH2) (2b). [Cp*RhCl2]2 (0.76 g, 1.23 mmol), 4-tBuC6H4SO2DPEN (1.0 g, 2.45 mmol), and NEt3 (0.68 mL, 4.9 mmol) were reacted in CH2Cl2 (50 mL) to give 2b (1.0 g, 60%). Crystals suitable for X-ray analysis were grown by slow diffusion of hexane into a concentrated solution of 2b in CH2Cl2. 1H NMR (500 MHz, CD2Cl2, 300 K, numbering as in Figure 2): δ 1.21 (9H, s, CMe3), 1.80 (15H, s, C5Me5), 3.47 (1H, br d, JHH ≈ 10 Hz, NHb), 3.69 (1H, ddd, JHH ) 3, 12, 14 Hz, H10), 3.90 (1H, br apparent t, JHH ≈ 11, 11 Hz, NHa), 4.07 (1H, d, JHH ) 11 Hz, H5), 6.59 (2H, d, JHH ) 7 Hz, H7), 6.66 (2H, t, JHH ) 8 Hz, H8), 6.71-6.74 (1H, m, H9), 6.85-6.87 (1H, m, H12), 6.96 (2H, d, JHH ) 10 Hz, H2), 7.08-7.14 (3H, m, H13 and H14), 7.23 (2H, d, JHH ) 9 Hz, H3). 13C (125 MHz, CD2Cl2, 300 K): δ 9.96

Half-Sandwich Rhodium Amido Complexes (s, C5Me5), 31.40 (s, CMe3), 69.75 (s, C5), 71.90 (s, C10), 99.48 (d, JRhC ) 9 Hz, C5Me5), 124.60 (s, C2), 126.96 (s, C9), 127.53 (s, C8), 127.84 (s, C12), 127.86 (s, C3), 128.76 (s, C13 or C14), 128.96 (s, C13 or C14), 129.35 (s, C7), 139.72 (s, C6 or C11), 140.12 (s, C6 or C11), 143.02 (s, C4), 152.53 (s, C1). 15N NMR (50.7 MHz, CD2Cl2, 300 K): δ -263.7 (d, JRhN ) 17 Hz, NH2), -208.6 (d, JRhN ) 20 Hz, NTs). MS (FAB, m/z): 681 ([MH+]). IR (KBr/cm-1): 3287 (νNH), 3233 (νNH). Anal. Calcd for C34H42ClN2O2RhS: C, 59.95; H, 6.22; N, 4.11. Found: C, 59.70; H, 6.25; N, 4.16. 1

H NMR (500 MHz, CD3CN, 300 K): δ 1.24 (9H, s, CMe3), 1.80 (15H, s, C5Me5), 3.64 (1H, br apparent t, JHH ) 11, 11 Hz, NHb), 3.78 (1H, ddd, JHH ) 3, 12, 14 Hz, H10), 4.00 (1H, d, JHH ) 11 Hz, H5), 4.42 (1H, br d, JHH ) 9 Hz, NHa), 6.63 (2H, d, JHH ) 7 Hz, H7), 6.68 (2H, d, JHH ) 7 Hz, H8), 6.73-6.76 (1H, m), 6.90-6.92 (2H, m), 7.04 (2H, d, JHH ) 9 Hz), 7.12-7.13 (3H, m), 7.27 (2H, d JHH ) 9 Hz). Cp*RhCl(4-FC6H4SO2NCHPhCHPhNH2) (2c). [Cp*RhCl2]2 (0.83 g, 1.35 mmol), 4-FC6H4SO2DPEN (1.0 g, 2.7 mmol), and NEt3 (0.75 mL,5.4 mmol) were reacted in CH2Cl2 (50 mL) to give 2c (1.35 g, 78%). Crystals suitable for X-ray analysis were grown by slow diffusion of hexane into a concentrated solution of 2c in CH2Cl2. 1H NMR (500 MHz, CDCl3, 300 K): δ 1.80 (15H, s, C5Me5), 3.54 (1H, br d, JHH ≈ 10 Hz, NHb), 3.70 (1H, ddd, JHH ) 2, 12, 14 Hz, H10), 3.84 (1H, br apparent t, JHH ≈ 11, 11 Hz, NHa), 3.96 (1H, d, JHH ) 11 Hz, H5), 6.62 (2H, d, JHH ) 9 Hz, H7 or H8), 6.65-6.68 (2H, m, H7 or H8), 6.78 (2H, apparent t, JHH ) 8, 8 Hz, H2), 6.82-6.85 (3H, m, H9 and H12), 7.10 (3H, m, H13 and H14), 7.40 (2H, dd, JHH ) 8 Hz, JHF ) 5 Hz, H3). 13C (125 MHz, CD2Cl2, 300 K): δ 9.96 (s, C5Me5), 69.85 (s, C5), 71.87 (s, C10), 94.92 (d, JRhC ) 8 Hz, C5Me5), 113.9 (C2,d, JFC ) 22 Hz), 127.09 (s, C9 or C12), 127.55 (s, C9 or C12), 127.66 (s, C7 or C8), 128.79 (s, C13 or C14), 128.96 (s, C13 or C14), 129.32 (s, C7 or C8), 130.79 (d, JFC ) 6 Hz, C3), 139.87 (s, C5 or C10), 139.94 (s, C5 or C10), 141.70 (s, C4), 163.43 (d, JFC ) 250 Hz,C1). 19F NMR (480 MHz, CD2Cl2, 300 K): δ -112.76 (s). 1H–15N HMQC NMR (50.7 MHz, CD2Cl2, 300 K): δ -263.8 (d, JRhN ) 16 Hz, NH2), -210.2 (d, JRhN ) 21 Hz, NTos). 1H-103Rh HMQC (22.18 MHz, CDCl3, 300 K): δ 2036.5. MS (FAB, m/z): 643 ([MH]+). IR (KBr/cm-1): 3292 (νNH), 3234 (νNH). Anal. Calcd for C30H33ClFN2O2RhS: C, 56.04; H, 5.17, N, 4.36. Found: C, 55.70; H, 5.39; N 4.39. 1 H NMR (500 MHz, CD3CN, 300 K): δ 1.80 (15H, s, C5Me5), 3.61 (1H, br apparent t, JHH ≈ 11, 11 Hz, NHb), 3.80 (1H, ddd, JHH ) 3, 12, 14 Hz, H10), 3.94 (1H, d, JHH ) 11 Hz, H5), 4.44 (1H, br d, JHH ≈ 8 Hz, NHa), 6.69 (2H, d, JHH ) 7 Hz), 6.75-6.79 (4H, m), 6.83-6.85 (1H, m), 6.89-6.91 (2H, m) 7.12-7.14 (3H, m), 7.44 (2H, dd JHH ) 9 Hz, JFH ) 5 Hz). 15 N-Labeled Cp*RhCl(TsNCHPhCHPhNH2) (2a-15N). [Cp*RhCl2]2 (36.0 mg, 58 µmol), TsDPEN (41.0 mg, 112 µmol), and NEt3 (33 µL, 237 µmol) were reacted in CH2Cl2 (5 mL), to give 15Nenriched 2a (54 mg, 71%). 1H NMR (500 MHz, CDCl3, 300 K): δ 1.80 (15H, s, C5Me5), 2.21 (3H, s, Me), 3.48 (1H, dd, JNH ) 72 Hz, JHH ) 10 Hz), 3.70 (1H, ddd, JHH ) 14, 12, 3 Hz), 3.86 (1H, d of apparent t, JNH ) 82 Hz, JHH ) 11, 11 Hz), 3.96 (1H, d, JHH )11 Hz), 6.63 (2H, d, JHH ) 7 Hz), 6.75 (2H, t, JHH ) 7 Hz), 6.80-6.85 (5H, m), 7.06-7.14 (3H, m), 7.28 (2H, d, JHH ) 7 Hz). 15 N{1H} NMR (50.7 MHz, CD2Cl2, 300 K): δ -263.6 (d, JRhN ) 14 Hz, NH2), -210.6 (d, JRhN ) 18 Hz, NTs). MS (FAB, m/z): 639 ([MH+] 14N/14N isotopomer), 640 ([MH]+ 14N/15N isotopomer), 641 ([MH]+ 15N/15N isotopomer).

Synthesis of [Cp*Rh(CO)(CONTsCHPhCHPhNH2)][Cl] (3a). 3a was prepared by adding CO (3 atm) to an NMR tube fitted with a Young’s tap containing a degassed solution of 2a in dry CD2Cl2, CD3OD, or a mixture of CD2Cl2/CD3OD (1%). 1H NMR (500 MHz, CD2Cl2, 300 K): δ 1.68 (s, 15H, C5Me5), 1.83 (s, 15H, C5Me5), 2.28 (s, 3H, Me), 2.31 (s, 3H, Me), 4.49-4.51 (m, 2H, CH), 5.04-5.09 (m, 2H, CH), 6.02 (d, JHH ) 6 Hz, 1H, NH), 6.07 (d, JHH ) 6 Hz, 1H, NH), 6.82-7.40 (m, 28H,

Organometallics, Vol. 28, No. 5, 2009 1445 aromatic), 7.56 (m, 1H, NH), 7.78 (d, JHH ) 8 Hz, 1H, NH). C NMR (75 MHz, CD2Cl2, 300 K): δ 9.06 (s, C5Me5), 9.30 (s, C5Me5), 21.16 (s, Me), 60.72 (s, CH), 60.84 (s, CH), 63.55 (s, CH), 64.10 (s, CH), 106.24 (d, JRhC ) 5 Hz, C5Me5), 106.55 (d, JRhC ) 5 Hz, C5Me5), 124.86 (s), 126.90 (s), 127.26 (s), 127.59 (s), 127.66 (s), 127.78 (s), 127.83 (s), 128.44 (s), 128.68 (s), 129.10 (s), 129.32 (s), 137.30 (s), 137.46 (s), 137.95 (s), 138.24 (s), 138.36 (s), 138.43 (s), 142.77 (s), 142.98 (s), 181.58 (d, JRhC ) 27 Hz, NCO), 181.75 (d, JRhC ) 27 Hz, NCO), 188.08 (d, JRhC ) 77 Hz, CO), 188.14 (d, JRhC ) 77 Hz, CO). 13C NMR (125 MHz, CD2Cl2, 300 K): δ 181.6 (d, JRhC ) 27 Hz, NCO), 181.7 (d, JRhC ) 27 Hz, NCO), 188.1 (d, JRhc ) 77 Hz, CO), 188.2 (d, JRhc ) 77 Hz, CO). MS (FAB, m/z): 603 ([M - 2CO]+). IR (CH2Cl2/cm-1): 2056 (νCO), 1600 (νCO). [Cp*Rh(tBuNC)(TsNCHPhCHPhNH2)][PF6] (4a). 4a was prepared by addition of tBuNC (13 mg, 0.16 mmol) to a solution of 2a (100 mg, 0.16 mmol) in CH3OH (5 mL), stirring for 1 h, removing the solvent under reduced pressure, and extracting into toluene. The addition of KPF6 (59 mg, 0.32 mmol) to the toluene solution resulted in the precipitation of a yellow solid. The yellow solid was extracted into THF, filtered, and crystallized from THF/ hexane to give 4a (102 mg, 76%) as yellow needles. 1H NMR (500 MHz, CD3CN, 300 K): δ 1.70 (s, 9H, tBu), 1.91 (s, 15H, C5Me5), 2.15 (s, 3H, Me), 3.75 (ddd, JHH ) 3, 12, 14 Hz, 1H, H10), 4.09 (br d, JHH ≈ 11 Hz, 1H, NHa), 4.15 (d, JHH ) 11 Hz, 1H, H5), 4.29 (br apparent t, JHH ≈ 11, 11 Hz, 1H, NHb), 6.60 (d, JHH ) 7 Hz, 2H, Ph), 6.65 (t, JHH ) 8 Hz, 2 H, Ph), 6.74 (d, JHH ) 8 Hz, 2H, H2), 6.74-6.76 (m, 1H, H9, or H14), 7.03-7.04 (m, 2H, Ph), 7.12 (d, JHH ) 8 Hz, 2H, H3), 7.18 (m, 3H, Ph). 13C NMR (125 MHz, CD3CN, 300 K): δ 10.33 (s, C5Me5), 21.50 (s), 30.64 (s, CMe3), 60.37 (s, CMe3), 71.74 (s, C10), 73.84 (s, C5), 102.26 (d, JRhC ) 7 Hz, C5Me5), 127.60 (s, Ph), 128.52 (s, Ph), 128.77 (s, Ph), 129.03 (s, C2), 129.33 (s, C3), 129.62 (s, Ph), 130.00 (s, Ph), 134.8 (d, JRhC ) 69, CN), 139.69 (s, Ph), 140.11 (s, Ph), 141.40 (s, C1 or C4), 141.91 (s, C1 or C4). 31P NMR (202 MHz, CD3CN, 300 K): δ -144.61 (sept, JFP ) 702 Hz). MS (ESI, m/z): 686 ([M+]). IR (KBr/ cm-1): 3336 (νNH), 3292 (νNH); 2212 (νCN). Anal. Calcd for C36H45F6N3O2PRhS: C, 51.99; H, 5.42; N, 5.05. Found: C, 51.68; H, 5.48; N, 4.99. [(Cp*Rh)2(µ-H)(µ-Cl)(µ-HCO2)][BPh4] ([5a][BPh4]). 2a (200 mg, 0.32 mmol) was dissolved in CH3OH (10 mL) with the aid of sonication, and HCO2H (150 mg, 3.26 mmol) was added via microsyringe. The solution turned from orange to red and was stirred for a further 15 min before stirring was stopped and NaBPh4 (220 mg, 0.64 mmol) was added as a solid, and the flask was swirled to afford mixing, causing a red precipitate to settle from the solution upon standing. The solid was removed by filtration, dried under reduced pressure, and crystallized from acetone/Et2O (1:1) at -20 °C to give [5a][BPh4] as red crystals (121 mg, 43%). Crystals suitable for X-ray analysis were grown from a solution of [5a][BPh4] in (CH3)2CO/Et2O (1:1) at -20 °C. 1H NMR (500 MHz, (CD3)2CO, 300 K): δ -8.59 (t, JRhH ) 27.4 Hz, 1H, Rh-H-Rh), 1.94 (s, 30H, C5Me5), 6.77 (t, JHH ) 7.2 Hz, 4H, BPh4), 6.92 (t, JHH ) 7.5 Hz, 8H, BPh4), 7.33-7.35 (m, 8H, BPh4), 7.46 (t, JHH ) 3.5 Hz, 1H, HCO2). 13C NMR (125 MHz, (CD3)2CO, 300 K): δ 10.09 (s, C5Me5), 98.93-98.99 (m, C5Me5), 122.29 (s, BPh4), 126.01-126.08 (q, JBC ) 3 Hz, BPh4), 137.07-137.08 (m, BPh4), 164.40-165.58 (q, JBC ) 49 Hz, BPh4), 173.84 (s, HCO2). IR (KBr/ cm-1): 1556 (νCO). MS (ESI, m/z): 557 ([M]+). Anal. Calcd for C45H52BClO2Rh2: C, 61.63; H, 5.98. Found: C, 61.21; H, 6.12. 13

Transfer Hydrogenation of d6-Acetone with d15-NEt3/Formic Acid Catalyzed by 2a. (S,S)-2a (4.4 mg, 6.8 µmol) was dissolved in CD3CN; d6-acetone (10 µL, 0.14 mmol) was added to the tube via microsyringe followed by d15-NEt3 (35 µL, 0.27 mmol). At each stage of addition, a 1H NMR spectrum was recorded. The sample was cooled to 250 K, HCO2H (26 µL, 0.680 mmol) was added to

1446 Organometallics, Vol. 28, No. 5, 2009 the tube, and spectra were recorded every 10 min for 1 h. The experiment was repeated at 300 K. Metathesis Reactions of 2a and 2c. Cp*RhCl(4-RC6H4SO2NCHPhCHPhNH2) was dissolved in the minimum volume of CH3OH (often with the aid of sonication), the relevant reagent was added as a solid (LiBr, KI, AgOTf) or liquid (tBuNC), and the solution was stirred for 1 h or more. The solvent was removed under reduced pressure and the residue extracted into CH3CN, filtered through Celite, and concentrated to dryness under vacuum. The product was recrystallized by layering a THF solution with hexane. Synthesis of Cp*RhBr(TsNCHPhCHPhNH2) (6a). 6a was prepared as described above from 2a (150 mg, 0.23 mmol), LiBr (21 mg, 0.24 mmol), and CH3OH (10 mL). After removal of CH3CN the product was crystallized from THF/hexane to give 6a (140 mg, 89%) as red crystals. Crystals suitable for X-ray analysis were grown by slow diffusion of hexane into a concentrated solution of 6a in THF. 1H NMR (500 MHz, CD3CN, 300 K, numbering as in Figure 2): δ 1.82 (s, 15H, C5Me5), 2.19 (s, 3H, Me), 3.75 (br apparent t, JHH ≈ 11, 11 Hz, 1H, NHa), 3.83 (ddd, JHH ) 3, 12, 14 Hz, 1H, H10), 4.14 (d, JHH ) 11 Hz, 1H, H5), 4.62 (br d, JHH ≈ 10 Hz, NHb), 6.65-6.71 (m, 4H, Ph), 6.76-6.77 (m, 1H, Ph), 6.80 (d, JHH ) 8 Hz, 2H, H3), 7.05 (m, 2H, Ph), 7.12-7.13 (m, 3H, Ph), 7.23 (d, JHH ) 8 Hz, 2H, H2). 13C NMR (125 MHz, CD3CN, 300 K): δ 10.41 (s, C5Me5), 21.55 (s, Me), 70.39 (s, C10), 72.55 (s, C5), 95.96 (d, JRhC ) 9 Hz, C5Me5), 127.47 (s, Ph), 128.31 (s, Ph), 128.88 (s, C2 and Ph), 129.13 (s, Ph), 129.20 (s, H2), 129.48 (s, Ph), 130.34 (s, Ph), 140.52 (s, C1), 140.96 (s, Ph), 141.08 (s, Ph), 143.07 (s, C4). IR (KBr)/cm-1): 3271 (νNH), 3210 (νNH). MS (FAB, m/z): 682 ([M+] 79Br isotopomer), 684 ([M+] 81Br isotopomer). Cp*RhI(TsNCHPhCHPhNH2) (7a). 7a was prepared as described above from 2a (250 mg, 0.34 mmol) and KI (57 mg, 0.34 mmol) in CH3OH (12 mL). The product was recrystallized from THF/hexane to give 7a (174 mg, 70%) as dark red crystals. Crystals suitable for X-ray analysis were grown by slow evaporation of a solution of 7a in CH3OH. 1H NMR (500 MHz, CD3CN, 300 K): δ 1.92 (s, 15H, C5Me5), 2.21 (s, 3H, Me), 3.83 (m, 1H, Ha), 3.84 (m, 1H, H10), 4.01 (d, JHH ) 11 Hz, 1H, H5), 4.35 (br d, JHH ≈ 9 Hz, 1H, Hb), 6.66-6.69 (m, 4H, Ph), 6.74-6.76 (m, 1H, Ph), 6.80 (d, JHH ) 8 Hz, H3), 6.92-6.94 (m, 2H, Ph), 7.12-7.13 (m, 3H, Ph), 7.21 (d, JHH ) 8 Hz, 2H, H2). 13C NMR (125 MHz, CD3CN, 300 K): δ 11.11 (s, C5Me5), 21.46 (s, Me), 71.29 (s, C10), 72.16 (s, C5), 96.12 (d, JRhC ) 8 Hz, C5Me5), 127.37 (s, Ph), 128.06 (s, Ph), 128.67 (s, C2), 128.87 (s, C3), 129.06 (s, Ph), 129.51 (s, Ph), 130.58 (s, Ph), 140.19 (s, C1), 140.33 (s, Ph), 141.22 (s, Ph), 144.46 (s, C4). IR (KBr/cm-1): 3278 (νNH), 3233 (νNH). MS (FAB, m/z): 731 ([MH]+). Anal. Calcd for C31H36IN2O2RhS: C, 50.97; H, 4.97; N, 3.83. Found: C, 50.45; H, 5.03; N, 3.61. [Cp*Rh(4-FC6H4SO2NCHPhCHPhNH2)][CF3SO3], [8c][OTf] and [8c-H2O][OTf]. [8c][OTf] was prepared as described above from 2c (500 mg, 0.78 mmol), AgCF3SO3 (200 mg, 0.78 mmol), and CH3OH (40 mL). The product mixture was filtered through Celite before the solvent was removed under vacuum. Compound [8c][OTf] (470 mg, 80%) was obtained as a purple powder. Crystals suitable for X-ray analysis were grown by slow diffusion of hexane into a concentrated solution of [8c][OTf] in CH3OH/toluene (1:1) but proved to be [8c-H2O][OTf]. 1H NMR (500 MHz, CD3CN, 300 K): δ 1.83 (s, 15H, C5Me5), 3.75 (ddd, JHH ) 3, 12, 14 Hz, 1H, H10), 3.95 (apparent t, 11 Hz,1H NHb), 4.15 (d, 11 Hz, 1H, H5), 4.50 (d, JHH ) 10 Hz, 1H, NHa), 6.63-6.68 (m, 4H, Ph), 6.70 (apparent t, JHH ) 8 Hz, JFH ) 8 Hz, 2H, H2), 6.76-6.79 (m, 1H, Ph), 7.06-7.08 (m, 2H, Ph), 7.16-7.17 (m, 3H, Ph), 7.31 (dd, JHH ) 11 Hz, JFH ) 5 Hz, 2H, H3). 13C NMR (125 MHz, CD3CN, 300

Blacker et al. K): δ 9.71 (s, C5Me5), 70.65 (s, C10), 72.64 (s, C5), 98.08 (d, JRhC ) 8 Hz, C5Me5), 115.10 (d, JFC ) 22 Hz, C2), 122.09 (q, JFC ) 316 Hz, CF3), 127.49 (s, Ph), 128.23 (s, Ph), 129.09 (s, Ph), 129.13 (s, Ph), 129.81 (s, Ph), 130.85 (d, JFC ) 10 Hz, C3), 139.59 (s, Ph ipso), 139.94 (s, Ph ipso), 142.09 (d, JFC ) 4 Hz, C4), 163.84 (d, JFC ) 243 Hz, C1). 19F NMR (470 MHz, CD3CN, 300 K): δ -79.2 (s, 3F, CF3), -113.0 (s, 1F, FC6H4). MS (ESI, m/z): 607 ([M]+). IR (KBr/cm-1): 3278 (νNH), 3246 (νNH). Anal. Calcd for [8cH2O][OTf], C31H35F4N2O6RhS2: C, 48.07; H, 4.55; N, 3.62. Found: C, 47.89; H, 4.51; N, 3.65. [Cp*Rh(tBuNC)(4-FC6H4SO2NCHPhCHPhNH2)][CF3SO3] (4c). 4c was prepared from 8c by adding tBuNC (11 mg, 0.13 mmol) to a solution of 8c (100 mg, 0.13 mmol) in CH3OH (3 mL), filtering, and removing the solvent under vacuum, to give 4c (46 mg, 55%) as a yellow powder. 1H NMR (500 MHz, CD3CN, 300 K): δ 1.70 (s, 9H, CMe3), 1.91 (s, 15H, C5Me5), 3.75 (ddd, JHH ) 3, 12, 14 Hz, 1H, H10), 4.12-4.18 (m, 1H, NHa), 4.15 (d, JHH ) 11 Hz, 1H, H5), 4.44 (br apparent t, JHH ≈ 12, 12 Hz, 1H, nHb), 6.63-6.68 (m, 4H, Ph), 6.70 (dd, JHH ) 8 Hz, JFH ) 8 Hz, 2H, H2), 6.76-6.79 (m, 1H, Ph), 7.04-7.07 (m, 2H, Ph), 7.17-7.18 (m, 3H, Ph), 7.26-7.28 (dd, JFH ) 5 Hz, JHH ) 9 Hz, 2H, H3). 13C NMR (125 MHz, CD3CN, 300 K): δ 10.39 (s, C5Me5), 30.68 (s, CMe3), 60.62 (s, CMe3), 71.80 (s, C10), 73.76 (s, C5), 102.26 (d, JRhC ) 7 Hz, C5Me5), 115.62 (d, JFC ) 23 Hz, C2), 122.81 (q, JFC ) 335 Hz, CF3), 128.01 (s, Ph), 128.75 (s, Ph), 129.14 (s, Ph), 129.69 (s, Ph), 130.20 (s, Ph), 131.44 (d, JFC ) 10 Hz, C3), 139.61 (s, Ph), 140.09 (s, Ph), 141.37 (d, JFC ) 4 Hz, C4), 164.0 (d, JFC ) 243 Hz, C1). 19 F NMR (470 MHz, CD3CN, 300 K): δ -79.23 (s, 3F, CF3), -112.63 (s, 1F, FC6H4). MS (ESI, m/z): 690 ([M]+). X-ray Crystallography. X-ray data were collected on a Bruker SMART Apex X-ray diffractometer for θ > 1.72° up to θ < 28.1-29.99°. The crystals were cooled to 110 K using an Oxford Cryosystems Cryostream. Diffractometer control, data collection, and initial unit cell determination were performed using SMART (v5.625 Bruker-AXS). Frame integration and unit-cell refinement were carried out with SAINT+ (v6.22, Bruker AXS). Absorption corrections were applied by SADABS (v2.03, Sheldrick). The structures were solved by direct methods using SHELXS-97 and refined by full-matrix least-squares using SHELXL-97.63,64 Hydrogen atoms bound to carbon were placed using a riding model and included in the refinement at calculated positions. Hydrogen atoms attached to nitrogen in 2c, 6a, and 7a were located on difference maps after all non-hydrogen atoms had been located and were refined isotropically (2c, 7a). Crystal data for complexes 2c, [5a][BPh4] · OEt2, 6a, 7a, and 8c-H2O are shown in Table 2.

Acknowledgment. We acknowledge the support of EPSRC and NPIL Ltd. We also appreciated discussions with Matthew Stirling, George Hodges, Anthony Haynes, and Richard Douthwaite. We thank David Taylor for recording the rhodium NMR spectra, and Richard Lindup for assistance with preparing the manuscript. We thank the reviewer for helpful criticism of the manuscript. Supporting Information Available: Cif files for 2c, [5a][BPh4] · OEt2, 6a, 7a, and 8c · H2O. This material is available free of charge via the Internet at http://pubs.acs.org. OM8009969 (63) Sheldrick, G. M. SHELXS-97, a program for crystal structure solution; Go¨ttingen University: Germany, 1997. (64) Sheldrick, G. M. SHELXL-97, a program for crystal structure determination; Go¨ttingen University: Germany, 1997.

Reactivity, Structures, and NMR Spectroscopy of Half-Sandwich

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